505 



The experiment was made with a piece of brown tourmaline 

 having a few opake streaks, procured from Mr. Darker of Lambeth. 

 It was placed in a graphite frame between two circular holes made 

 as above described in opposite sides of the bomb, the diameter of the 

 holes being about -^yths of an inch. On looking in at one of these 

 holes you could thus see through the tourmaline and the opposite 

 hole, or, in other words, see'quite through the bomb. An arrange- 

 ment was also made by which part of the tourmaline might be viewed 

 with the graphite behind it. 



The apparatus thus prepared was heated to a red or yellow heat 

 in the fire, placed on a brick in a dark room, and the tourmaline 

 viewed by a polariscope which Mr. Gassiot kindly lent me. The 

 following was the appearance of the experiment : 



Without the polariscope the transparent parts of the tourmaline 

 were slightly less radiant than the field around them. When the 

 polariscope was used, the light from the transparent portions of the 

 tourmaline was found to vary in intensity as the instrument was 

 turned round. No change of intensity could be observed in the 

 light radiated by the opake streaks of the tourmaline, or by the 

 graphite. 



The light from the transparent portions was therefore partially 

 polarized. The polariscope was then brought to its darkest position, 

 and a light from behind allowed to pass through the tourmaline. 

 The light was distinctly visible in this position, but by turning round 

 the polariscope about 90 it became eclipsed. The mean of four 

 sets of experiments made the difference between the position of dark- 

 ness for the two cases 88 1. It appears, therefore, that the light 

 radiated by the tourmaline was partially polarized in a plane at right 

 angles to that which was transmitted by it. It was also ascertained 

 that the light from the tourmaline which had the graphite behind it 

 gave no trace of polarization. 



